1. Field of the Invention
The present invention relates to a biological sensor for non-invasively measuring a concentration of a material in a living body by the utilization of a plurality of lights having different wavelengths the light absorbing characteristics of which in the living body are different from one another. More particularly, the invention relates to a biological sensor which may easily be attached to the living body with high accuracy in measurement.
2. Related Art
The technique on the pulse oximeter is known for the technique for non-invasively and continuously measuring a concentration of a material in a living body by the utilization of different light absorbing characteristics of a plurality of wavelengths of lights. The measuring technique calculates an oxidation difference of hemoglobin in a blood of a living body by using a ratio of intensities of two wavelengths of lights whose light absorbing characteristics are different. It is known that the technique using a plurality of wavelengths of lights may also be available for calculating another material in the living body. An example of this is a technique to calculate a concentration of indo-cyanine green (ICG) in a blood by using three wavelengths of lights. A pulse oximeter using two wavelengths will be discussed, for explanation, in the description to follow. Also in the measurement using three or more wavelengths, however, the same thing is correspondingly applied to the basic technique, mainly the detection technique on the probe, a kind of biological sensors.
The pulse oximeter has rapidly been prevailed in the medical field in the world since the principle of the pulse oximeter disclosed in JP-B-53-26437. Presently, the pulse oximeter is one of the parameters indispensable for monitoring a condition of a patient, and it is a fairly general measuring item. The advantageous feature of the pulse oximeter resides in that it is able to measure an oxygen saturation in an arterial blood by a non-invasive measuring method.
The principle of the pulse oximeter is based on the fact that hemoglobin contained in the red blood cell in the blood changes its color when it is combined with oxygen, and hence, the arterial oxygen saturation can be obtained by measuring the light absorbing characteristic of the hemoglobin. Actually, one and the same sample being in the same state is measured by using two wavelengths of lights which are different in light absorbency, in the same condition. In this case, a ratio of the measurement results corresponds to the oxygen saturation in one-to-one correspondence. Lights having two wavelengths of about 660 nm and about 900 nm are used for the pulse oximeter measurement. A change of the light absorbancy of the light of 660 nm, caused by the oxygen saturation of the hemoglobin, is much larger than that of the light of 900 nm.
Specifically, as shown in FIG. 7, when a thickness D of a sample is changed by xcex94D by a pulsation, and transmitted light I is attenuated by xcex94I, a change xcex94A of the light absorbancy is given by
xcex94Axe2x89xa1log[I/(Ixe2x88x92xcex94I)]=ECxcex94Dxe2x80x83xe2x80x83(1)
Changes xcex94A1 and xcex94A2 of the light absorbancy (where 1 and 2 affixed to letters A indicate 660 nm and 900 nm) are measured and a ratio "PHgr" of them is calculated, then we have
"PHgr"xe2x89xa1xcex94A1/xcex94A2=E1/E2xe2x80x83xe2x80x83(2)
Thus, we have the light absorbancy ratio.
FIG. 8 shows in block and schematic form a basic construction of a pulse oximeter. A light source consists of two light emitting diodes (LEDs), and those LEDs alternately and rapidly flicker when receiving a signal from an oscillator (OSC). Light passes through a living tissue and reaches a photo diode (PD) which in turn converts an intensity of transmitted light into a corresponding current. The current is converted into a voltage, amplified, and split according to two wavelengths by a multiplexer (MPX). As a result, electric pulse signals of each wavelengths are obtained. Those pulse signals are logarithmically converted and the pulsating components of the signals are extracted through a band-pass filter (BPF). Each extracted one is a pulsating component xcex94A of an attenuation of an object to be measured.
The pulsating component xcex94A is defined by
xcex94Axe2x89xa1log(Iout/I)≈AC/DCxe2x80x83xe2x80x83(3)
"PHgr"xe2x89xa1xcex94A1/xcex94A2≈(AC1/DC1)/(AC2/DC2)xe2x80x83xe2x80x83(4)
In the above expressions, AC and DC are, respectively, an amplitude of the pulsating component and a stationary component of the transmitted light. Thus, "PHgr" as a ratio of the pulsating components of the lights of the two wavelengths can be obtained by using the division in place of the logarithmic process.
Finally, the oxygen saturation can be obtained by mathematically processing "PHgr" or by using a conversion table for the "PHgr".
To cause the computer to compute an exact oxygen saturation, the conditions required at the measuring location of the measured object through which the lights are transmitted may be concluded from the principle of the pulse oximeter such that xe2x80x9cthe lights having the wavelengths to be detected must be transmitted through the same location and travel an equal distance, and further must be influenced by the same living tissue and bloodxe2x80x9d.
Let us consider the current measurement on the basis of the conditions required for the pulse oximeter xe2x80x9cthe lights having the wavelengths to be detected must be transmitted through the same location and travel an equal distance, and further must be influenced by the same living tissue and bloodxe2x80x9d.
In an early stage of a probe for the pulse oximeter, an incandescent light bulb was used as a light source, and optical filters corresponding to the wavelengths were provided at two light receiving portions, whereby information on the two wavelengths was obtained. FIG. 9 shows an example of the early probe where an earlobe is used for an object to be measured.
In FIG. 9, an ear piece 2 forming an ear oximeter 1 is constructed with a light emitting portion 3 and a light receiving portion 4, which are optically coupled to each other, and a holder 6 including an appropriate slide which supports those elements and is able to adjust a distance between them and a fixing mechanism 5. A light emitting portion 3 contains a light source 7 therein, and a couple of photo transistors 8 and 9 are attached to the inside of the light receiving portion 4. The photo transistors 8 and 9 receives lights of wavelengths 660 nm and 900 nm, respectively. The ear piece 2 interposes an earlobe 12 with cushions 10 and 11 attached to the opposed surfaces of the light emitting portion 3 and the light receiving portion 4.
Thereafter, the LEDs are introduced into the probe, so that the probe size becomes small. This kind of probes as shown in FIGS. 10 and 11 have been used. The probe is attached to a finger, and the light emitting portion and the light receiving portion are provided in the upper and lower attaching portions of the probe. Those light emitting and receiving portions are oppositely disposed, and a tissue is interposed between them. Light transmitted through the tissue is detected. This type of the probe will be referred to as a xe2x80x9ctransmit typexe2x80x9d of probe.
Another probe is shown in FIG. 12. As shown, a light emitting portion and a light receiving portion are secured onto a surface of a flexible member while being spaced a fixed distance (e.g., 10 mm). Lights scattered and reflected in the inner side of the fingertip or the like are measured. This type of the probe is referred to as a xe2x80x9creflection typexe2x80x9d of probe.
The transmit type of the probe generally consists of a clip type of probe as shown in FIGS. 10 an 11, and a winding type of probe, which utilizes adhesion, as shown in FIGS. 13 and 14. FIG. 14 is a cross sectional view showing a structure of the FIG. 13 probe. In FIGS. 13 and 14, reference numeral 14 is a flexible tape member for holding the light emitting portion 3 and the light receiving portion 4 and to be applied to a finger 15. FIG. 17 shows an electric wiring between the light emitting portion 3 and the light receiving portion 4. For this type of the probe, a photo-electric sensor probe disclosed in JP-B-2-20252 is known.
The clip type of the probe is large in size. The light emitting portion and the light receiving portion are fixed while being confronted with each other. In the FIG. 10 probe, those portions are opened and closed by a hinge 16. Therefore, the optical axis of the probe is little shifted. In FIG. 11, those portions slide vertically, so that the probe is free from the shift of the optical axis.
This teaches that the light detection required for the pulse oximeter can be performed in an ideal condition. On the other hand, the winding type of the probe shown in FIGS. 13 and 14 has widely been spread in the form of xe2x80x9cdisposable usexe2x80x9d in which cleaning and sterilizing of the probe are not required before and after the probe is attached to the object to be measured, in order to overcome the large size of the probe which is the disadvantage of the clip type of the probe and to reduce the cost to manufacture. Particularly, for the measurement of a neonate or a pediatric patient, the probe used must be small and clean. This also promotes such use of the probe.
The attaching of the conventional winding type of the probe shown in FIGS. 13 and 14 will be analyzed in detail.
FIGS. 15A to 15C diagrammatically show a probe when it is attached to fingers being different in size. In those figures, the light emitting portion 3, the light receiving portion 4 and a tape member 14 as a support structural member as an adhesive are illustrated in part. In each figure, the light emitting portion 3 was fixed to a lunula ungues of a nail bed of a finger 15, and the tape member was wound on the finger. If necessary, the light receiving portion 4 may first be fixed thereto. Since one and the same probe is used, a distance between the supports of the light emitting portion 3 and the light receiving portion 4 remains unchanged, as a matter of course. Accordingly, as seen, a position on the inner side of the finger tip portion to which the light receiving portion 4 is put is different finger by finger.
FIG. 16 shows the corrected attachment of the probe to the finger shown in FIG. 15A in which the tape member 14 is relatively long, which the correction is made such that the light emitting portion 3 and the light receiving portion 4 are oppositely disposed. The light emitting portion 3 and the light receiving portion 4 may be attached to locations that are oppositely disposed, but a sag is made by an extra sticking portion of the tape member. Actually, a wiring cable 16 of the light receiving portion 4 is contained in this portion as shown in FIGS. 17 and 18. This makes it impossible to remove this sag. When a patient moves, the patient rubs against the slack portion of the tape member to needlessly stimulate the patient. The needless stimulation will cause a rash of a weak skin of a neonate or a pediatric patient. FIG. 17 is an exploded view showing the probe, FIG. 18 is a perspective view showing the assembled one, and FIG. 19 is a sectional view showing the probe when it is attached to the finger 15. In those figures, reference numeral 17 is a cord which bundles a wiring cable 18 for supplying power source to the light emitting portion 3 and a wiring cable 16 for lading a signal out of the light receiving portion 4. Further, reference numeral 19 is a tape member for winding support structural members 13 and 14 around the finger and fixing them to the latter.
How a shift of a detecting position of the light, which is caused depending on a state of the attachment of the light emitting and receiving portions in the winding type of the probe, affects a measuring accuracy will be considered on xe2x80x9cthe component theoretically estimated as an error factor of the pulse oximeterxe2x80x9d in light detected by the light receiving portion.
Firstly, xe2x80x9clight not attenuated by bloodxe2x80x9d, called xe2x80x9cleak lightxe2x80x9d, may be enumerated. An example of the leak light is LED light B or C leaking along the surface of a skin or into a space between the skin and the support structural member 14. The FIG. 16 case including a slack and the FIGS. 15(a) and 15(c) cases are the very typical examples in which the leak light relatively increases. According to the expression 4, the leak light is added as xe2x80x9clight having no pulsationxe2x80x9d that is the DC component of each wavelength.
"PHgr"xe2x89xa1xcex94A1/xcex94A2≈(AC1/(DC1+R1))/(AC2/(DC2+R2))xe2x80x83xe2x80x83(5)
In the above expression (5), R1 and R2 are leak light components.
These leak lights are not only transmitted through the blood per se but also is affected by reflection, absorption and the like by a skin tissue whose light absorption is different for each wavelength of light and on the tape surface. The leak lights, while varying in intensity, are added to the DC components as seen from the expression (5). As a result, the calculation result contains an error. For reference, an attenuation characteristic of fowl from which blood is removed, which is a representative example of a state of a tissue having no blood, is shown in FIG. 21. With regard to the light attenuation by the tissue where light is not attenuated by the blood, absorbing light A at 660 nm or therearound is apparently different from absorbing light B at 900 nm or therearound.
Another error cause is such that light scattering in the tissue changes depending on the wavelength of light, and hence a location in the tissue through which light has transmitted changes. A light intensity distribution changes depending on an angle with respect to the center of the light emitting element in each LED device. Where 0xc2x0 is set at a position just above the surface of the light emitting element, an axis of light passes through 0xc2x0 in both the longitudinal and cross sectional planes.
A light sensitivity of the light receiving element per se of the PD device also changes in value depending on the angle. Also in this case, an axis of light, also called an optical axis passes through the position of 0xc2x0 in both the longitudinal and cross sectional planes. It is safety to say that the optical axis is an axis providing substantially proper characteristics, although those are somewhat different by the lens effect of a transparent resin covering the element. Some examples of it are shown in FIGS. 22(a) and 22(b) in which dotted lines and solid lines indicate directivities of the light emitting element for different wavelengths. The light intensity difference described above is limited to the light intensity difference by an angle in a space. Actually, light, which is derived from a location of the living body to which the light is projected, is transmitted through the living tissue while at the same time is scattered by the tissue. In the case of the light projected through the surface of a skin, its scattering in the tissue changes depending on the wavelength. Accordingly, its distribution configuration is not uniform. As in an example of FIG. 23, a light distribution configuration varies depending on the wavelength of light. The longer the wavelength is, the harder the scattering of light is. IR light of 900 nm tends to be less scattered. Further, the scattering of light is caused by cells and cell membranes of blood cells and the like in the tissue. The scattering change dependent on the wavelength is more distinguished for a finger of a pediatric patient whose light transmitting thickness is thin. Accordingly, the intensities of lights having different wavelengths and the locations in the tissue through which the lights are transmitted are deviated depending on the location of the light receiving portion. This possibly causes an error in the light detection based on the measurement principle, and the error will lead to an error in the calculation.
As described above, in the measurement by the pulse oximeter, it is necessary to detect the lights transmitted through the same location and tissue and attenuated by the same living tissue and blood. The light receiving portion must detect lights of two wavelengths in the conditions which are equal to each other as much as possible. For this reason, it is desirable to transmit lights through the thickest location of the tissue and to align the light emitting and receiving portions with the optical axis as a position where the scattering of the light having one wavelength and that of the light having the other wavelength are both maximized. In a state as shown in FIG. 24 where the optical axis is shifted, the scattering of light in the tissue changes, so that light whose attenuation is deviated is detected, and there is a chance of increasing the leak light forming the error factor described already.
Those facts imply that in the measurement of a thin finger which is thin in thickness and tends to cause non uniform scattering of light, in particular the finger of a neonate or a pediatric patient, an exact coincidence of it with the optical axis is required. Further, in the finger of a neonate or a pediatric patient, a distance between the light emitting and receiving portions is shorter than that in the adult""s one. The influence by the leak light is not negligible. Accordingly, it is implied that an error of the measurement, which is influenced by a shift of the optical axis, will increase. The reflection type of the probe is a typical example of the probes suffering from the great optical axis shift. This type of the probe is disadvantageous in securing exactness in measurement, although it has an advantage of easy attachment.
The winding type of the probes in which the distance between the light emitting and receiving portions is set to two kinds are currently marketed in the light of production cost in manufacturers and the inventory management in actual hospitals. etc. An operator can select the probe suitable for the patient from his experience to certain degree. However, the probe will be not always fit to the patient since the distance between the light emitting and receiving portions is fixed in value. Accordingly, erroneous measured values will be produced highly possibly. In other words, we recognized such a structural problem of the disposable type probe, which can be manufactured at low production cost, that the light emitting portion and the light receiving portion are fixed to one and the same tape (support structural member).
When a conventional probe in which the light emitting and receiving portions are assembled into a unit form as shown in FIG. 18 is attached to a person to be measured by use of a long strip-like tape member 19, care must be taken so as not to mistakenly attach the probe to an incorrect location of the measured person. Further, there is the possibility that the long tapes 19 incorrectly adhere to one another. To avoid this, the operator must carefully apply the probe to the measured person. Particularly when the patient is not cooperative to attach the probe, when the tape is passed through between a target finger and another finger, it is frequently attached to another finger. Generally, as the attaching and detaching of the tape of the probe are repeated, an adhesive force of the probe tape becomes weaker. Accordingly, when the tape is attached to an incorrect location of the patient, the probe fails to exhibit its own performance.
Where the light emitting portion 3 and the light receiving portion 4 are interposed between the tape members 13 and 14, holes 14a and 14b as light transmission windows are formed in the tape member 14 to be applied to a location to be measured. Those windows allow a sufficient amount of light to pass therethrough for reception and transmission. With provision of the holes 14a and 14b, a step is formed between the hole 14a and its peripheral part by a height of the tape member 14, as shown in FIG. 25 showing a portion including the light emitting portion 3 in cross section. This step will create a partial insertion 15a of the tissue at the probe attaching location. As a result, ischemia or stasis will occur at a contour of the tissue along the edge of the hole, i.e., a circumferential edge 15b. 
It is readily understood that transmission of light through such a location of the tissue as to receive a physical stress from exterior should be avoided in order to secure an accurate measurement of a patient""s condition. A thin transparent member, such as nonwoven fabric may be interposed between the holes 14a, 14b as the light transmission windows and the skin at the probe attaching location. In this case, the member, together with the skin, is often put into the hole because of its flexure.
An approach to remove the steps by filling the holes 14a and 14b with transparent material, e.g., resin, is disadvantageous in complexity of the manufacturing process, long time taken for the manufacturing, and hence poor manufacturing efficiency. Further, unless the elastic characteristic of the material is exactly the same as that of the tape members 13 and 14 around the material, its edge portions corresponding to the boundaries between it and the holes 14a and 14b form a step. Specifically, if an elasticity of the filling material is stronger than that of the tape member around it, the whole filling portion is pressed against the skin. If the elasticity of the filling material is weaker than that of the tape member around it, a partial insertion of the skin is caused.
The present invention has been made in view of the above circumstances, and has an object of providing a biological sensor which is easy to be attached and has a high measuring accuracy.
According to the present invention, there is provided a biological sensor for non-invasively measuring a concentration of a material in a living body by detachably attaching the biological sensor to a skin surface of the living body, the biological sensor having a light emitting portion and a light receiving portion for detecting lights which is emitted from the light emitting portion and transmitted through the living body, wherein the light emitting portion and the light receiving portion may be attached to the opposed locations of a skin surface of the living body, and the light emitting portion and the light receiving portion are firmly fixed to separate support structural members, respectively.
In the biological sensor, the support structural members include pairs of tape members which interpose the light emitting portion and the light receiving portion, and the tape members covering a light emitting surface of the light emitting portion and a light receiving surface of the light receiving portion are transparent.
In the biological sensor, at least one of the support structural members is symmetrically configured with respect to a line on which the light emitting portion or the light receiving portion lies.
In the biological sensor, at least one of the support structural members is symmetrically configured with respect to a line on which the light emitting portion or the light receiving portion lies, and the support structural member includes wing portions extending to both sides.
In the biological sensor, the support structural members are each symmetrically configured with respect to a line on which the light emitting portion or the light receiving portion lies respectively and the support structural members each include wing portions extending to both sides.
In the biological sensor, at least one of the support structural members is furnished with a mark being oriented in a direction in which the support structural member is attached to the living body location.
In the biological sensor, the light emitting portion and the light receiving portion are separately formed. Accordingly, those may easily be attached to a skin surface of a living body in a state that their optical axes are coincident with each other. As a result, a concentration of a material in a living body may accurately be measured. In the biological sensor, the tape members to be brought into contact with the patient""s skin are transparent. With this feature, there is no need of the light transmission holes, which are formed at the measuring locations of the conventional tape members. In this respect, a seamless structure is realized. Accordingly, when the biological sensor is fit to the patient for a long time, the patient""s skin is not damaged and the invasion to the skin is minimized.
In the biological sensor, the support structural member or members are each symmetrically configured with respect to a line on which the light emitting portion or the light receiving portion lies. Further, in the biological sensor, one of the support structural members is furnished with a mark and both of the support structural members are each furnished with a reference line being oriented in a direction in which the support structural member is attached to the living body location. Therefore, the support structural members may easily be attached to correct locations of a living body. The furnished mark also indicates that the biological sensor is used for one of a neonate or a pediatric patient. The furnished reference line is referred to when the light emitting portion and the light receiving portion are attached in a state that the optical axes of the light emitting portion and the light receiving portion are coincident with each other.